In general, a spark plug used for igniting an internal combustion engine such as a gasoline engine includes a center electrode, an insulator provided around the center electrode, a metallic shell provided around the insulator, and a ground electrode (also called “outer electrode”) which is attached to the metallic shell and forms a spark discharge gap in cooperation with the center electrode.
There has been known a spark plug in which an electrode tip made of a noble metal such as platinum or iridium is joined to a spark discharge portion of the ground electrode so as to improve the resistance to spark erosion and the resistance to oxidation erosion. The electrode tip is joined to the ground electrode by means of resistance welding. Specifically, in a state in which one end portion (base end portion) of the ground electrode is joined to a forward end portion of the metallic shell, the other end portion (distal end portion) of the ground electrode and the electrode tip are sandwiched from opposite sides by the forward end surfaces of two welding electrodes so as to apply a pressure thereto. In such a state, a voltage is applied between the welding electrodes, whereby the ground electrode and the electrode tip are welded together (see, for example, Japanese Patent Application Laid-Open (kokai) No. H7-22157 “Patent Document 1”). Also, the ground electrode is joined to the metallic shell by means of resistance welding. Specifically, the metallic shell is supported by one welding electrode, and the ground electrode is chucked by the other welding electrode. The ground electrode and the metallic shell are sandwiched between the two welding electrodes so as to apply a pressure thereto. In such a state, a voltage is applied between the welding electrodes, whereby the ground electrode and the metallic shell are welded together.